1. Field of the Invention
The present invention relates to image precessing method and apparatus applicable to a color facsimile, color image file and the like.
2. Related Background Art
Adaptive discrete cosine transformation (ADCT) coding system has been known as a highly efficient coding. Such system is summarized as follows:
1) Input image is converted into YCrCb signals and, considering human visual characteristics, is sampled at a ratio of Y:Cr:Cb=4:1:1 or 4:2:2. A description will be given below with respect to a case of using the 4:1:1 ratio.
2) The image is divided into N.times.N (8.times.8 for example) pixel blocks, and each N.times.N block is subjected as a unit to a cosine transformation.
3) The transformed coefficient is quantized by a suitable quantization step (for example using a quantization matrix as shown in FIGS. 3A-3C).
4) For a DC (direct current) component, the difference in DC component from the previous block is used, and the resulting value is converted into Huffman code.
5) For an AC (alternating current) component, a conversion into one-dimensional series is first made by means of zigzag scan (as shown in FIG. 7), and Huffman coding is performed on the basis of non-0 coefficient and the length of zero-run up to that point.
6) The above described steps 2) to 5) are performed with respect to each of the Y, Cr, and Cb components, and as shown in FIG. 13 one block each of Cr, Cb components is transmitted for every four blocks of Y component (such one block of Cr, Cb of course, is one which corresponds to the four blocks of Y component).
At the decoding side, decoded image is obtained by performing the inverse process of the above described Steps 1) to 6).
In the conventional example as described, however, there has been a problem that, since the same quantization matrix is used for images of dissimilar statistical characteristics, an optimal quantization may not be performed, thereby lowering the quality of picture image.
Further, since in the above described conventional example four blocks of Y component and one block each of Cr, Cb components are sent in a combination, it is necessary to provide memories for 6N (2N for each of R, G, B, i.e., 48) lines if they are to be decoded by lines at the decoder side.
For this reason, there has been a problem in that the circuit structure at the decoding side is increased in size.
Furthermore, such problem similarly exists also at the encoding side.